A multichannel television system is used in the United States that is commonly referred to as the BTSC system (Broadcast Television System Committee) in which stereo composite signals are transmitted with a pilot signal at a frequency of 15.734 khz. The pilot signal is FM modulated onto a 4.5 mhz sound carrier signal and has an amplitude magnitude that corresponds to a five khz (plus or minus 10%) deviation from the sound carrier. Receivers that are designed to receive and reproduce television stereo sound usually include circuitry that determines whether the signal is mono or stereo and, based on this determination, the television receiver switches between mono and stereo modes of operation. One way to make the stereo/mono determination is to monitor the presence of the pilot signal which would indicate that a stereo audio signal is being received. However, the noisy environment of most television receivers and the presence of harmonics of horizontal scan frequency in the television signal may interfere with the accurate determination of whether the pilot signal is present. Noise and harmonics are a special problem in most cable television systems, and, unless precautions are taken, the television receiver may be fooled into constantly switching between the two modes (stereo and mono) of operation.
A similar problem arises in connection with the detection of a second audio program (SAP) in a television signal. In the BTSC system, the SAP signal has a carrier signal appearing in the composite baseband at a frequency of 78.67 khz with an amplitude corresponding to a 15 khz (plus or minus 10%) deviation from the sound carrier which has a frequency of 4.5 mhz. In order to detect a SAP signal, one technique is to detect the presence of the SAP carrier frequency signal in the audio baseband. Again, however, noise and harmonic signals may fool the detector that is attempting to sense the SAP carrier.